rename rpc_token to TransportToken

[lixinqi]

没有改变逻辑，仅仅重命名类和变量。

[github-actions]

Speed stats:

GPU Name: GeForce GTX 1080 

PyTorch resnet50 time: 138.2ms (= 6908.0ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 125.9ms (= 6294.8ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
Relative speed: 1.10 (= 138.2ms / 125.9ms)

PyTorch resnet50 time: 83.4ms (= 4169.7ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 72.7ms (= 3634.0ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
Relative speed: 1.15 (= 83.4ms / 72.7ms)

PyTorch resnet50 time: 55.4ms (= 2769.7ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 48.4ms (= 2420.3ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
Relative speed: 1.14 (= 55.4ms / 48.4ms)

PyTorch resnet50 time: 49.7ms (= 2487.4ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 46.4ms (= 2317.6ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
Relative speed: 1.07 (= 49.7ms / 46.4ms)

PyTorch resnet50 time: 38.8ms (= 1942.4ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 42.8ms (= 2138.2ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
Relative speed: 0.91 (= 38.8ms / 42.8ms)